Sludge dewatering apparatus and process

ABSTRACT

A process and apparatus for sludge dewatering including gravity screening and capillary dewatering wherein an endless belt of a screen-like material and an endless belt of a capillary material are selectively functionally utilized separately and in close proximity to effectuate the steps of the process including a repetitive dewatering of the belt as it becomes saturated. The repetitive dewatering step proceeds with the screen-like belt supporting the sludge while the capillary belt is separately compressed, or otherwise dewatered. The process and apparatus further includes a final sludge dewatering section wherein the screen-like belt and the capillary belt are similarly separated.

United States Patent [1 1 Beristain et al.

[ SLUDGE DEWATERING APPARATUS AND PROCESS [75] Inventors: Charles D.Beristain; Barrie K.

Nixon; Robert J. Kiefer, all of Richmond, Va.

[73] Assignee: Westinghouse Electric Corporation,

' Pittsburgh, Pa.

22 Filed: Sept. 29, 1971 21 Appl. No.: 184,675

[56] References Cited UNITED STATES PATENTS 10/1972 Levin et a1 210/3868/1969 Pastoors et a1. 210/400 4/1968 Ishigaki 210/401 [111 3,774,760Nov. 27, 1973 Primary ExaminerSamih N. Zaharna Assistant Examiner-RobertH. Spitzer Att0rneyA. T. Stratton et al.

[57] ABSTRACT A process and apparatus for sludge dewaten'ng includinggravity screening and capillary dewaten'ng wherein an endless belt of ascreen-like material and an endless belt of a capillary material areselectively functionally utilized separately and in close proximity toeffectuate the steps of the process including a repetitive dewatering ofthe belt as it becomes saturated. The repetitive dewatering stepproceeds with the screen-like belt supporting the sludge while thecapillary belt is separately compressed, or otherwise dewatered. Theprocess and apparatus further includes a final sludge dewatering sectionwherein the screen-like belt and the capillary belt are similarlyseparated.

9 Claims, 1 Drawing Figure BACKGROUND OF THE INVENTION This inventionpertains to processes and apparatus for dewatering sludges and moreparticularly to such systems for dewatering sludgesprimarily through theapplication of capillary force produced by porous materials.

Sludges include those mixtures of particulate matter and water generallyattained in the treatment of waste waters including domestic sewage andindustrial waste or in water treatment plants. The sludges may compriseprimary sludge, secondary or activated sludge, digested sludge, alumsludge, lime sludge and mixtures of the above sludges.

Sludges have been conventionally dewatered by treatment systemsincluding vacuum filtration, centrifugation, and pressure filtration.Each of the systems suffer from inherent deficiencies. For example,vacuum filtration requires an elevated concentration of particulatematter in the sludge for efficient operation. Centrifugation requires alarge power consumption, and pressure filtration requires excess washingto maintain any respectable treating capacity.

Capillary belts have been subject to repeated experimentation since theydo not suffer from the particularized deficiencies associated with theabove systems. Capillary belts are evaluated as to the volume of sludgefeed they can handle (hydraulic loading), final solids concentration,and the quality of the supernatant. These parameters are all related tothe capillary force a belt can develop.

The capillary force developed is basically a function of the number ofpores and their size and the wetting characteristics of the liquid.Although belt materials are at least theoretically known which canmaximize capillary force, above a certain level the force developedbecomes self-defeating in the sense that the water cannot be removedfrom the belt. Any capillary belt utilized must effect a compromisebetween its ability to absorb water and its ability later to dischargethis absorbed'water by the application of some externally applied force.

A possible solution to the inherent limitation on useful capillaryforce, might be to reuse each section of the belt much in the manner inwhich a housewife uses a sponge, i.e., absorb liquid, squeeze liquidfrom sponge, absorb more liquid, etc. However, this approach can also beself-defeating since pressing the sludge into-the belt can blind thepores of the belt with particles. Moreover, disturbing the sludge layerwhile it still contains considerable water can be detrimental to thecontinued dewatering of the sludge.

SUMMARY OF THE INVENTION In accordance with this invention, an endlessbelt of porous material capable of developing an adequate capillaryforce is cooperatively associated with an endless belt of screen-likematerial. The system basically tering the capillary belt repetitively asit becomes saturated. This repetitive dewatering also takes place withthe screen-like belt separated from the capillary belt.

Initially the sludge which is rather dilute and contains much free wateris fed to the screen-like belt alone. It has been found that with anhydraulic head of approximately 2 inches that 20 to 45 percent of thefree water can be drained from the sludge in 20 to 30 seconds. Thepartially dewatered sludge is then carried by this screen-like belt to alocation where the screen-like belt and the capillary belt are placed insufficiently close proximity that the capillary belt and aqueous liquidproduce a capillary force which removes further water from the sludge.As the capillary belt becomes saturated the screen-like belt isseparated therefrom and the capillary belt is treated either bycompression or by the application of a vacuum to draw a substantialquantity of water therefrom. The screen-like belt and the capillarybelts are then once again brought into close proximity so that thecapillary force may remove still further water from the sludge. Thesesteps are repeated a sufficient number of times to remove a highpercentage of the water from the sludge. At the end of this repetitiveprocess the screen-like member is once again separatedfrom the capillarybelt and the capillary belt is again treated to remove water therefrom.The screen-like belt wraps around its return roller a foot or so alongthe machine length. A compression roller is associated with the returnroller to further dry the sludge. The surface of the compression rolleris such that the now partially dry sludge preferentially sticks to theroller surface and may be scraped therefrom into a receptacle via theusual knife-edge.

The belt and screen then proceed to cleaning locations where water underpressure is sprayed on the surface of the capillary belt and thescreen-like belt, respectively, from which locations the belts areconveyed to the inlet side of the apparatus and the process is repeated.

DESCRIPTION OF THE DRAWINGS FIG. I is a schematic view of a sludgedewatering apparatus in accordance with the principles of thisinvention.

DESCRIPTION OF A PREFERRED EMBODIMENT The following description is of anillustrative embodiment of the invention and it will be apparent tothose working in the art that many modifications may be made theretowithin thescope of the invention. Some of these modifications areincluded with the description of the preferred illustrative embodiment.

One example of a suitable sludge is activated sludge, as for examplefrom a secondary clarifier of a domestic waste purification system, orsuch activated sludge mixed with primary feed such as that in municipalsewage systems. Such sludge is normally very dilute and may have aparticulate content as low as 0.1 percent.

The basic system 10 has as its operative elements a screen-like belt 12and a capillary belt 14 of a porous material; both of which elementswill be described in greater detail later in this application. Thescreen-like belt 12 and the capillary belt 14 take the form of endlessbelts so as to facilitate the operation of the continuous process inwhich they are cooperatively associated. The apparatus basicallyincludes a free water dewatering section 16, a capillary dewateringsection 18, and a final dewatering section 20.

Initially the sludge feed which is normally very dilute and containsmuch free water which is easily removable is fed onto the screen-likebelt 12 via a constant level reservoirlike device 22. As can be seen inFIG. 1, the screen-like member 12 is utilized without the capillary belt14. it has been found that the greater the hydraulic head on this freewater dewatering section 16 the more rapidly free water will drainthrough the screen-like member 12. However, the layer of sludge fed ontothe capillary belt 14 must not be so thick as not to be readilydewatered. Means such as a container having a side wall illustrated at23 are provided to permit a head to be created on member 12. A 2 inchhead has been found to be sufficient to remove to 45 percent of thisfree water in a matter of 20 to 30 seconds. However, higher heads may befound to be more efficient for some purposes.

An underflow weir 24, of conventional construction such as an adjustablewall means illustrated by reference character 25 adjustably mounted oncontainer wall 23 having its free end 27 and spaced from screen 12, maybe utilized at the end of the dewatering section 16. The spacing betweenscreen 12 and weir end 27 serves to maintain the desired head and alsoto set the thickness of the sludge feed layer as the screen-like member12 comes in contact with the capillary member 14. As can be seen, thecapillary member 14 is fed underneath the screen-like member 12 over alarge rotating drum 26.Thismanner of feed allows the settled layer ofsludge created in the free water dewatering section to come in contactwith the capillary belt 14 without disturbance. It has been found thatany mixing or disturbing of the sludge at the entrance to the capillarybelts 14 is detrimental to the dewatering process. In the capillarydewatering section 18, the screen-like belt 12 and the capillary belt 14must be in close contact during those times in which a capillary forceis desired to draw water from the sludge layer. The close proximity ofthe belts 12 and 14 may be achieved by supporting these belts at aplurality of roller locations. As will be explained, these rollerlocations also contain compression rollers which remove water from thecapillary belt 14 as it becomes saturated.

An important feature of this invention is the construction of thecapillary belt 14 which must produce sufficient capillary force toproperly dewater the sludge while said force remains weak enough thatthe belt may be easily dewatered by compression rollers, or the otherknown dewatering means. The capillary belt 14 is preferably constructedfrom relatively inert synthetic fibers, e.g., nylon fibers, each fiberin this example being of approximately 4 9% inches in length. Thedesired weight of the belt is approximately 7.1 oz. 1 ft. It is desiredthat the finished belt have between 85 and 95 percent voids. in thisexample, to achieve this goal, the individual fibers are approximately43 microns in thickness. Preferably belt thickness is in the range ofone-half to nine-sixteenths inch. The sheet is properly manufactured ifit is capable of transferring water therethrough in a relatively shorttime under the influence of gravity alone when the amount of water, orsimilar liquid, on its top surface is approximately that amount whichwould overflow the sheet.

Because of the inherent limited hydraulic capacity of capillary beltmaterials, in accordance with this invention the capillary belt 14 isrepeatedly dewatered as it becomes saturated during sludge dewateringruns. As

can be seen from the illustrative embodiment, a plurality of dewateringsections designated by the numerals 28, 30, 32, 34 and 35 are providedin the capillary dewatering section 18 for removing water from thecapillary belt 14 as it becomes saturated. Although five such sections28,30, 32, 34 and 35 are shown, a greater or fewer number of suchsections may be utilized depending upon the rate of saturation of thecapillary belt 14. It has been found desirable to place the dewateringlocations 28, 30, 32, etc. closer together at the beginning of thecapillary dewatering section 18 than at the end. This is due to the factthat as the sludge becomes dryer, it becomes progressively moredifficult to remove water therefrom and thus requires more time for thecapillary belt to become saturated. The dewatering sections 28, 30, 32,and 34 include guide rollers 36 and 38 and compression rollers 40 and42. Dewatering of the belt 14 takes place with the screen-like member 12proceeding over the rollers 36, 40 and 38 while the capillary belt 14proceeds over the roller 36, between 14 once again comes into closeproximity with the screenlike belt 12. 40 and 42, where compression anddewatering take place, and then back over roller '38, where thecapillary belt 12.

The final capillary belt 14 dewatering section'35 also includes a rollerset similar to sets 28, 30, 32 and 34 except that the roller 38 isreplaced by a relatively large return roller 44 around which thecapillary belt wraps to begin its return passage.

The screen-like belt 12 wraps around a return roller 46 which isdisplayed some distance in the belt moving direction from the capillarybelt return roller 44.

A final dewatering of the layer of sludge previously dewatered by thefree water dewatering section 16 and the capillary dewatering section 18takes place as the screen-like member 12 proceeds over the screen returnroller 46. One favorable technique evolved for effectuating this finaldewatering is to situate a compression roller above and inspring loadedrelationship with the return roller 46; which compression roller isdesignated by the numeral 50. The top compression roller 50 in thisexample is chain driven and is positioned so that a line through itsaxis of rotation is forward of a similar line through the axis of thereturn'roller 46. This displacement of the'axis of rotation of therollers 46 and 50 effects a lead angle'for the sludge as it comes incontact with the compression roller 50. If the top roller 50 is situateddirectly above the return roller 46, the sludge normally tends tocollect in front of the top roller 50, and the roller 50 ceases toremove sludge; It has further been found that if the top roller 50 ismade of a hard material which is finished to provide a smoothnon-wetting surface, the compressed sludge will preferentially stick tothe compression roller 50. This leaves the screen-like belt 12 muchcleaner than would otherwise be the case. The supernatant is alsocleaner, and the sludge may be removed from the compression roller 50 bysuch means as a knife edge, as designated by the numeral 52. Thescreen-like belt 12 is preferably constructed of inert synthetic wovenfiber which provide openings therebetween which allow aqueous liquid topass therethrough but not the usual range of particulate matter of whichsludge is comprised.

Another approach to the final dewatering stage is to utilize the rollerconfiguration of FIG. 1 but to allow these rollers to act on both thescreen-like member 12 and the capillary belt member 14 while same are incontact. Using this approach, there is a'slight reduction in capitalcost of the equipment, however, a disadvantage of this approach is thatsome of the sludge material is forced through the screen 12 and into thecapillary belt 114. This means that if the belt material 14 is notwashed frequently, it may become blinded by the sludge particles andthis could seriously affect its hydraulic capacity.

The final dewatering stage may also take the form of an imperviousendless belt positioned above the screen 32 and capillary belt 14. Oneadvantage of using an impervious belt section is that it can beinfinitely adjusted so that any lead angle can be effected; as is knownto the art. Such an endless belt is let over a series of rollers; eachsuccessive roller being closer to the screen and belt combination andthus affecting a greater amount of compression.

A partial vacuum may also be used to accomplish a final dewatering. Atfirst glance, this may appear to be merely the addition of vacuumfiltration after a capillary dewatering stage. However, as thosefamiliar with the vacuum filtration art will know, a major problem, withthe practice of such art on such materials as activated sludge is thatthe fines in the feed material tend to clog and blind the filtermaterial. However, a dewatering section l8,'in accordance with thisinvention, builds up a precoat of sludge on the screen 12 such thatfines cannot be drawn into the openings of the filter screen H2.

The capillary belt M after proceeding over the return roller Ml passesover additional rollers 54 and 56 to a high pressure spray cleaninglocation 58 where the belt 14 is sprayed on its clean side and thenreturns for another pass via roller 26. The screen-like belt 12similarly proceeds via rollers 59 and 60 to a cleaner 62 and then viarollers 64, 66 and 68 to the front end of the apparatus lib. The beltsl2 and 14 can be driven in any conventional manner, for example a chainand sprocket arrangement can be used to drive all the friction and dragpoints, i.e., by direct coupling to the rollers. The aqueous liquidwhich is separated from the sludge is preferably collected in threeseparate receptacles 7t), 72,-and 74 having drain lines 76, 78 and 80,respectively. The effluent from these drain lines can be directed to theplant effluent or to further treatment apparatus depending upon itsparticulate content and other known parameters.

The relatively dry sludge is collected in a receptacle 82 and may eitherbe burned or used for land fill, as is known to the art.

We claim:

I. A process for dewatering a flowable material which includes suspendedsolids and water comprising the steps of:

feeding said flowable material onto the top surface of a moving endlessscreen belt having openings therein so sized as to retain substantiallyall of the suspended solids;

allowing the free water in the flowable material to drain through thebelt;

conveying said flowable material; having been partially dewatered, whilestill. on the surface of said moving screen belt into sufficiently closeproximity with the top surface of a moving capillary belt having poresso sized as to develop a capillary force with respect to the remainingwater in said flowable material as to further dewater the same;

repeatedly diverting the capillary belt away from the moving screenbelt;

squeezing the absorbed water out of the capillary belt each time it ismoved away from the screen belt; and

transferring the substantially dewatered flowable material from thescreen belt to a roller for subsequent discharge into a container.

2. The process of claim ll including squeezing the layer of solidsformed on the screen belt after it is separated from the capillary beltto further remove water therefrom.

3. The process of claim 1 including directing a high pressure waterspray against the said top surface of the screen belt and against thebottom surface of the capillary belt after removal of solids from thescreen belt for further removing any solids adhering to said belts; and

squeezing any remaining sprayed water from said belts.

4. Apparatus for removing water from a flowable material comprising:

a base supporting spaced upstanding members;

first rotatable means mounted on each of said members;

an endless porous screen belt extending between the members andremovably mounted on said spaced rotatable means and arranged such thatthe rotatable means moves the belt between said members in an upper runforwardly in a horizontal plane, and in a lower run in a reversedirection;

flowable material feed means adjacent one of said members for feedingflowable material onto said screen belt which allows free water in theflowable material to drain through the belt pores;

second rotatable means mounted on each of said members between saidfirst rotatable means;

an endless capillary belt made of relatively inert synthetic fibershaving pores therein of a size sufficient to develop a capillary forcecapable of absorbing water from the flowable material while still havingthe ability to later release such water by the application of anexternal force;

said capillary belt being mounted on said second rotatable means whichmoves the capillary belt in an upper run and horizontally underneath thescreen belt with its upper surface in contactwith the lower surface ofthe screen belt thereby effecting by capillary force, the transfer ofadditional water from the flowable materal on the porous screen belt tothe capillary belt; and 1 capillary belt dewatering means mounted onsaid base beneath the capillary and screen belts for periodicallyremoving the absorbed water from the capillary belt during the time thebelts move horizontally in the upper run between said members.

5. Apparatus according to claim 4 wherein said flowable material feedmeans comprising a container having an inlet and an outlet through whichflowable material is discharged onto said screen belt;

side walls on said container extending parallel to said belt forcontaining the flowable material on said belt; and flowable materiallevelling means on said container for maintaining the flowable materialat a predetermined level, thereby causing water in said flowablematerial to freely fall through the screen belt before the screen beltcontacts the capillary belt.

6. Apparatus according to claim 4 wherein the capillary belt dewateringmeans includes at least one combined belt support and dewatering device,the belt support portion being mounted to contact the inner surface ofthe capillary belt, a belt diverting portion in said dewatering devicewhich directs the capillary belt downwardly from the horizontal planefor passage through the dewatering device which squeezes water from thebelt.

7. Apparatus according to claim 4 wherein said capillary belt dewateringmeans comprises multiple combined belt support and dewatering devices,each of said device including a pair of spaced rollers positioned tosupport the screen and capillary belts as they move in said horizontalplanes;

a pair of vertically disposed belt squeezing rollers between the supportrollers, said squeezing rollers being spaced from each other a distanceless than the thickness of the capillary belt, the support and squeezingroller arrangement being such that the capillary belt moves over the topof one support roller then downwardly for movement between the squeezingrollers which squeezes water from the belt, and then over the top of theother support roller to resume movement in said horizontal plane.

8. Apparatus according to claim 7 wherein the feed means is mounted onone of said members and above the first rotatable means thereon and in aposition to discharge flowable material onto the screen belt, so thatfree water in the flowable material is permitted to flow through thescreen from the time the material is discharged onto the screen beltuntil the latter contacts the capillary belts.

9. Apparatus according to claim 8 wherein the first rotatable means onsaid base opposite the feed means is spaced a horizontal distance fromthe second rotatable means thereon to permit the screen belt to separatefrom the capillary belt;

said first rotatable means comprising a compression set of rollersthrough which the screen belt moves for further dewatering the flowablematerial and simultaneously transferring the flowable material to one ofthe compression rollers; and

scraper means adjacent said compression rollers for removing soliddewatered material from said screen belt.

2. The process of claim 1 including squeezing the layer of solids formedon the screen belt after it is separated from the capillary belt tofurther remove water therefrom.
 3. The process of claim 1 includingdirecting a high pressure water spray against the said top surface ofthe screen belt and against the bottom surface of the capillary beltafter removal of solids from the screen belt for further removing anysolids adhering to said belts; and squeezing any remaining sprayed waterfrom said belts.
 4. Apparatus for removing water from a flowablematerial comprising: a base supporting spaced upstanding members; firstrotatable means mounted on each of said members; an endless porousscreen belt extending between the members and removably mounted on saidspaced rotatable means and arranged such that the rotatable means movesthe belt between said members in an upper run forwardly in a horizontalplane, and in a lower run in a reverse direction; floWable material feedmeans adjacent one of said members for feeding flowable material ontosaid screen belt which allows free water in the flowable material todrain through the belt pores; second rotatable means mounted on each ofsaid members between said first rotatable means; an endless capillarybelt made of relatively inert synthetic fibers having pores therein of asize sufficient to develop a capillary force capable of absorbing waterfrom the flowable material while still having the ability to laterrelease such water by the application of an external force; saidcapillary belt being mounted on said second rotatable means which movesthe capillary belt in an upper run and horizontally underneath thescreen belt with its upper surface in contact with the lower surface ofthe screen belt thereby effecting by capillary force, the transfer ofadditional water from the flowable materal on the porous screen belt tothe capillary belt; and capillary belt dewatering means mounted on saidbase beneath the capillary and screen belts for periodically removingthe absorbed water from the capillary belt during the time the beltsmove horizontally in the upper run between said members.
 5. Apparatusaccording to claim 4 wherein said flowable material feed meanscomprising a container having an inlet and an outlet through whichflowable material is discharged onto said screen belt; side walls onsaid container extending parallel to said belt for containing theflowable material on said belt; and flowable material levelling means onsaid container for maintaining the flowable material at a predeterminedlevel, thereby causing water in said flowable material to freely fallthrough the screen belt before the screen belt contacts the capillarybelt.
 6. Apparatus according to claim 4 wherein the capillary beltdewatering means includes at least one combined belt support anddewatering device, the belt support portion being mounted to contact theinner surface of the capillary belt, a belt diverting portion in saiddewatering device which directs the capillary belt downwardly from thehorizontal plane for passage through the dewatering device whichsqueezes water from the belt.
 7. Apparatus according to claim 4 whereinsaid capillary belt dewatering means comprises multiple combined beltsupport and dewatering devices, each of said device including a pair ofspaced rollers positioned to support the screen and capillary belts asthey move in said horizontal planes; a pair of vertically disposed beltsqueezing rollers between the support rollers, said squeezing rollersbeing spaced from each other a distance less than the thickness of thecapillary belt, the support and squeezing roller arrangement being suchthat the capillary belt moves over the top of one support roller thendownwardly for movement between the squeezing rollers which squeezeswater from the belt, and then over the top of the other support rollerto resume movement in said horizontal plane.
 8. Apparatus according toclaim 7 wherein the feed means is mounted on one of said members andabove the first rotatable means thereon and in a position to dischargeflowable material onto the screen belt, so that free water in theflowable material is permitted to flow through the screen from the timethe material is discharged onto the screen belt until the lattercontacts the capillary belts.
 9. Apparatus according to claim 8 whereinthe first rotatable means on said base opposite the feed means is spaceda horizontal distance from the second rotatable means thereon to permitthe screen belt to separate from the capillary belt; said firstrotatable means comprising a compression set of rollers through whichthe screen belt moves for further dewatering the flowable material andsimultaneously transferring the flowable material to one of thecompression rollers; and scraper means adjacent said compression rollersfor removing solid dewatered material from said screen belt.